a. Field Of The Invention
The invention relates to a device for detecting environmental gamma radiation and radon gas concentrations. In particular, the invention is directed to a simple, low cost device for measuring the concentration of radon gas in homes and measuring the external radiation exposure from gamma-ray sources such as surrounding soil and building materials.
b. Description Of The Prior Art
Radon gas (.sup.222 Rn) is a decay product of the element radium (.sup.226 Ra) which is found in soils and rocks throughout the United States and the world. Radon can diffuse through cracks in rocks and through soil pores and enter the breathable atmosphere. Structures such as homes and other buildings can "trap" the .sup.222 Rn inside them because of typical low air ventilation rates. Concentrations of radon gas can possibly rise to high levels. High levels and their effects are known from the medical histories of hundreds of uranium miners who in many cases were exposed to high levels of the gas during the course of their work experience.
Although average household levels are probably orders of magnitude less than mine levels, the risk at these lower concentrations for developing lung cancer may not necessarily be insubstantial, due to the long time that people spend indoors and sometimes spend living in the same home.
With the threat of lung cancer definitively linked to exposure to .sup.222 Rn, it becomes necessary to determine where levels of the radon gas are acceptable and where they represent a significant health risk. This can only be done through actual measurement. .Many devices for use in mines have been developed but such arrangements are unsuited for measuring the indoor household atmosphere. In general, known radon gas detectors are large, unreliable and noisy because they employ air pumps which consume electric power and are prone to mechanical breakdown without frequent service or maintenance. Other measuring devices measure the radon gas concentration at a given moment but do not consider known diurnal variations in radon gas levels, and these fluctuations may often be significant Such "instantaneous" monitors must be run repetitively so that accurate averages may be derived, necessarily introducing many possible sources for erroneous measurement. Accurate average annual exposure to radon is essential for lung cancer risk estimation. Other devices measure levels of the radioactive decay products of the .sup.222 Rn itself, but these generally require expensive electronics and again provide only instantaneous measurement.
In particular, known devices for obtaining instantaneous radon gas samples include the Lucas flask, the two-filter method, and pulse ionization chambers. The Lucas flask is a small glass, metal or plastic chamber having a flat transparent bottom. The flask walls are opaque, and on the inside are coated with zinc-sulfide (ZnS(Ag)) phosphor. Air is permitted to enter the flask, and after a short period of exposure, alpha particles emitted by .sup.222 Rn, RaA and RaC' impinge on the zinc-sulfide phosphor thus causing the phosphor to scintillate. These scintillations are then counted using a photomultiplier tube (PMT) and sophisticated electronics to provide a measure of the radon gas concentration within the flask. While providing sensitivity as low as 0.1 pCi/liter, the Lucas flask is fragile, not easily transportable, and difficult for an untrained person to use.
The two-filter method involves drawing a large sample of air through a tube by using a high volume pump. The first filter removes radon daughters at the tube inlet, and the second filter collects a fraction of the radon daughters created within the tube at the tube outlet. Alpha activity of the outlet filter can then be related to the .sup.222 Rn concentration in the sampled air. This device requires a portable high volume air pump, and a portable alpha particle counter. Also, the sensitivity of the device depends primarily on the tube dimensions, flow rate and length of sampling time.
Pulse ionization chambers are sophisticated, accurate measuring systems requiring a good deal of support equipment. Thus, although extremely accurate and sensitive, they are not in any way suitable to measurements in the field.
For the continuous measurement of radon concentration present in the air, the continuous Lucas flask, continuous two-filter method, and diffusion/electrostatic radon monitors (types I and II) are known. The type I diffusion/ electrostatic monitor is a relatively small device for indoor environmental measurements. Radon passively diffuses into a hemispherically shaped enclosure formed of wire mesh supporting a foam filter material, which prevents entry of radon daughters into the enclosure. A negative electrode at the bottom of the enclosure is coated with zinc-sulfide (Zn-S(Ag)) phosphor. Positively charged radon daughters are collected on the eletrode where they decay. The alpha particles impinge on the phosphor, producing scintillations which may be detected by a photomultiplier tube (PMT) and then counted over a preset time interval with any conventional electronic equipment. While the type I device eliminates the need for the noisy and unreliable air pump, the device is subject to a known "memory" effect (due to the decay product RaC' produced by RaA, RaB and RaC in one count interval, which "adds" to the alpha count in the subsequent count period). This makes the measurements inaccurate over periods of time during which radon concentration varies.
A known type II diffusion/electrostatic radon monitor uses passive diffusion (filtered) to provide a sampling volume into which only radon, but not radon daughters, may enter. Inside the volume is a highly negatively charged electret surface which draws newly formed radon daughters away from the volume walls, the walls once again being coated with zinc-sulfide phosphor. Resulting scintillations are detected by a photomultiplier tube (PMT) and are counted. While an improvement over the type I diffusion/electrostatic monitor, this device still requires delicate counting electronics and the PMT, both of which require power supplies. For a one hour counting period, this device may have sensitivity as low as 0.03 pCi/liter of .sup.222 Rn.
Another general class of known monitoring device is the "integrating" monitor, in which a signal from the varying concentration of radon gas or its products is accumulated and then averaged over the exposure period to obtain a mean concentration. Known devices and techniques for integrating radon monitors include the technique, the "MYLAR bag technique," the diffusion/electrostatic integrating radon monitor, the diffusion/electrostatic integrating radon monitor using X-ray film, the activated carbon radon monitor, and the solid state nuclear track detector (SSNTD).
The "MYLAR bag technique" involves filling a bag through a precision needle valve with a battery-operated pump, over a period of about 48 hours. The MYLAR bag is returned to the laboratory for analysis of its contents. This method is not suitable for long term use, and the pump/needle valve arrangement is prone to frequent breakdowns.
A diffusion/electrostatic integrating radon monitor known as the Passive Environmental Radon Monitor (PERM) is described by Breslin, A. J., George, A. C., "An Improved Time-Integrating Radon Monitor," NEA Specialist Meeting On Personal Dosimetry and Area Monitoring Suitable for Radon and Daughter Products, Paris, Nov. 20-22, 1978. This device is similar to the continuous type I and II monitors described above, but the PMT is replaced by a signal integrating thermoluminescent dosimeter (TLD). Radon diffuses through a bed of silica gel positioned above the wide end of a metal funnel. The funnel end is covered covered by a wire mesh and filter paper to prevent entry of ambient radon daughters. The TLD. is placed at the bottom (narrow end) of the metal funnel, atop a brass electrode held at a high negative voltage by a battery. The metal funnel is electrically connected to the positive battery terminal. Positively charged radon daughters are attracted to the TLD where they deposit their energy of decay. Another TLD is provided to measure background gamma-radiation signal, which must be subtracted from the signal obtained from the radon-detecting TLD. The silica gel must be replaced frequently in conditions of high relative humidity, or the calibration is in error.
The X-ray film monitor is similar to the continuous type I diffusion/electrostatic monitor, except that the PMT and counter are replaced by X-ray film, which "records" each scintillation for later analysis.
The activated carbon monitor uses a sealed canister filled with activated carbon. When the canister is opened, radon gas is adsorbed; the canister is then resealed after a relatively brief period of time, for example, three days. The gamma-ray emissions from radon daughters trapped on the activated carbon may then be counted. This device is sensitive to humidity changes, and also has the drawback of variable accuracy, since it is possible for radon to desorb from the carbon during the sampling period.
Finally, the SSNTD involves counting radiation damage tracks left by alpha particles emitted by daughters RaA and RaC'. The alpha particles penetrate dielectric (detector) materials, such as cellulose nitrate, cellulose acetate, or a carbonic acid diester, leaving tracks which can be made optically visible by chemical etching, for subsequent counting. While advantageously passive in operation, inexpensive and small in size, the SSNTD is not generally highly accurate due to inherent difficulties in track counting. At low environmental levels, track counting is especially difficult due to presence of flaws in the detector material which appear as tracks, variations caused by the handling and storage history of the particular detector, and variability in the etching process for enhancing track visibility.
The use of electrostatically charged dielectric materials, such as polymers including polyurethane, polyethylene and fluorocarbons, to concentrate and collect positively charged radon daughters is also known. In particular, a TLD has been placed on top of a TEFLON electret in order to measure radon concentration, as described by Kotroppa, P., Dua, S. K., Pimpale, N. S., Gupta, P. C. et al., "Passive Measurement of Radon and Thoron Using TLD or SSNTD On Electrets," Health Phys. 43:399-404 (1982).
Others have investigated the use of electrets for passive radon daughter dosimetry. Phillips, C. R., Khan, A., Leung, H., Development of Diffusion-Based Radon Daughter Dosimeters, Research Report, Atomic Energy Control Board of Canada, INFO-0112 (1983); Khan, A., Phillips, C. R., "Electrets for Passive Radon Daughter Dosimetry," Health Phys. 46:141-49 (1984).
Despite the substantial work of others skilled in this crowded art and the long-felt need for a compact, silent, low-cost and portable radon detector, none of the aforementioned devices have proven satisfactory for the various reasons mentioned. Thus, it has not yet been feasible to undertake large-scale radon measurement programs in private residences and commercial buildings that would be required to map geographic regions requiring reduction of radon and radon daughter concentrations.